(1) Field of the Invention
The present invention relates to a Faraday rotator wherein an external magnetic field is applied such that the direction of magnetization of a Faraday element is tilted toward the light beam direction in order to reduce the temperature dependence of the Faraday rotation angle. Specifically, this invention relates to a Faraday rotator formed by a combination of three or more Faraday elements. This Faraday rotator is applicable, for example, to various kinds of optical devices that utilize the Faraday effect, such as optical attenuators and the like.
(2) Description of the Related Art
In optical communication systems, optical attenuators for controlling the amount of transmitted light, optical isolators for transmitting light only in one direction, and the like are necessary. Into these are built Faraday rotators for rotating the plane of polarization of the transmitted light. Additionally, Faraday rotators are used for various kinds of optical devices such as optical switches, optical circulators, optical filters, optical equalizers and the like.
For example, a variable optical attenuator is, in practice, sometimes necessary in a system with an optical amplifier. To be specific, in an erbium doped optical fiber amplifier or the like, a variable optical attenuator is used to maintain a constant output level within a range of required input levels. For this variable optical attenuator, the present applicant has proposed a variable optical attenuator wherein, with a configuration having no mechanical moving parts, the Faraday rotation angle is changed by changing the current applied to an electromagnet, and the attenuation is determined by the setting of this rotation angle (refer to the specification of Japanese Patent No. 2815509).
For Faraday elements to be built into Faraday rotators used for variable optical attenuators as mentioned above, Bi (bismuth)-substituted rare earth iron garnet single crystal film (LPE film) produced mainly by the LPE method (liquid phase epitaxial method) and the like have been used in recent years. That is because there is an advantage in that the LPE film has a larger Faraday rotation coefficient than YIG (yttrium iron garnet) single crystal due to the contribution of Bi.
However, this Bi-substituted rare earth iron garnet single crystal film has a deficiency in that there is a large temperature dependence of the Faraday rotation angle. Consequently, there has been a problem in that the temperature dependence of Faraday rotators also becomes large, and the temperature characteristics of devices such as variable optical attenuators and the like which are made up using such Faraday rotators is large.
Accordingly, the present applicants have proposed a technique wherein the temperature dependence of the Faraday rotation angle is reduced by applying an external magnetic field such that the variation of the Faraday rotation angle due to temperature dependence of the angle formed between the direction of magnetization and the light beam direction of a Faraday element, and the variation of the Faraday rotation angle due to temperature dependence of the Faraday effect are counterbalanced (refer to Japanese Unexamined Patent Publication No. 11-249095). Furthermore, this proposal also discloses a technique in that, when forming a Faraday rotator by combining a plurality of Faraday elements, by arranging each Faraday element according to a required standard and not in a random crystal orientation manner, the temperature dependence of the Faraday rotator is improved. To be specific, the details are that, for example, when three Faraday elements are used, by setting the crystal orientations of two Faraday elements in the same direction, and the crystal orientation of the remaining one Faraday element in a different direction, the temperature dependence of the Faraday rotation angle can be reduced.
However, with the proposed Faraday rotator mentioned above, only the crystal orientations of a plurality of Faraday elements are specified, and the order of each Faraday element, whose crystal orientation has been specified, is not specified. Therefore, if the magnetic field applied to each Faraday element becomes non-uniform state, there is a possibility that the temperature dependence of the Faraday rotation angle cannot be fully reduced. This is especially important when considering the construction of a Faraday rotator device. That is to say, since the electromagnet for controlling the Faraday rotation angle occupies the largest mounting space in the Faraday rotator, the size of the electromagnet needs to be minimized. However, since miniaturization of this electromagnet can easily cause non-uniformity in the magnetic field applied to each Faraday element, it is possible that the temperature dependence of the Faraday rotation angle cannot be fully reduced.
The present invention addresses the aforementioned points, with the object of providing a small size Faraday rotator which is easy to produce and enables reliable reduction of temperature dependence of the Faraday rotation angle, by defining the crystal orientation and the arrangement order of each Faraday element when a Faraday rotator is composed of three or more Faraday elements.
Here, for example, in a polarization independent optical isolator described in Japanese Unexamined Patent Publication No. 6-3622 and the like, a technique is disclosed in that a magnetic field is applied externally such that directions of magnetization of two optical path areas in a Faraday rotator are opposed to each other. However, since the object of the aforementioned invention is to realize an optical isolator which has high isolation and does not depend on a polarization, and the invention does not contain a technique such as for defining the crystal orientation of a Faraday element, it is different from the present invention.
To achieve the aforementioned object, according to the present invention, there is provided a Faraday rotator comprising three or more Faraday elements arranged along an optical axis direction, and a magnetic field applying device for applying an external magnetic field to each of the Faraday elements such that the direction of magnetization is tilted toward the optical axis direction, wherein each of the Faraday elements is arranged such that each crystal orientation is perpendicular to a light beam direction, and the crystal orientations of adjacent Faraday elements are opposed to each other.
With such a configuration, each Faraday element is magnetized by an external magnetic field applied by the magnetic field applying device, and a polarization plane of polarized light transmitted through each Faraday element is rotated. At this point, the external magnetic field is applied such that the direction of magnetization of each Faraday element is tilted toward the optical axis direction, whereby the temperature dependence of the Faraday rotation angle is reduced. Furthermore, by arranging each Faraday element such that the crystal orientations of adjacent Faraday elements are opposed to each other, then even though the component of the external magnetic field perpendicular to the optical axis direction becomes non-uniform state, an influence caused by this state is counterbalanced and reduced by a pair of adjacent Faraday elements. Hence the temperature dependence of the Faraday rotation angle can be reliably reduced. This also enables miniaturization of the Faraday rotator and an improvement in productivity.
Furthermore, with the aforementioned Faraday rotator, the magnetic field applying device may have a first magnetic field applying section for applying a magnetic field to each of the Faraday elements in a parallel direction to the optical axis direction, and a second magnetic field applying section for applying a magnetic field to each of the Faraday elements in a perpendicular direction to the optical axis direction. To be specific, the arrangement may be such that the first magnetic field applying section magnetically saturates each Faraday element using a fixed magnetic field from a permanent magnet, and the second magnetic field applying section controls the direction of magnetization of each Faraday element using a variable magnetic field from an electromagnet.
With this configuration, the arrangement is such that the direction of magnetization of each Faraday element is tilted toward the optical axis direction by a composite magnetic field of the first and the second magnetic field applying sections.
Furthermore, it is preferable that the magnetic field applying device of the aforementioned Faraday rotator applies an external magnetic field in a direction such that the magnitude of the total variation of a variation of the Faraday rotation angle due to temperature dependence of the angle between the direction of magnetization and the light beam direction of each Faraday element, and a variation of the Faraday rotation angle due to temperature dependence of the Faraday effect, is less than or equal in absolute terms to the magnitude of the total variation of the Faraday rotation angle due to temperature dependence of the Faraday effect of each Faraday element.
With such a configuration, the variation of the Faraday rotation angle due to temperature dependence of the angle between the direction of magnetization and the light beam direction of each Faraday element, and the variation of the Faraday rotation angle due to temperature dependence of the Faraday effect are counterbalanced, and temperature dependence of the Faraday rotation angle is reduced
Moreover, as a specific example of each Faraday element, (RBi)3(FeM)5O12 or (RBi)3Fe5O12 (where R is one or more elements selected from rare earth elements including yttrium, and M is one or more elements that can be substituted for iron) produced by the liquid phase epitaxial method, or Y3Fe5O12 may be used.
Furthermore, an optical attenuator may be constructed using the Faraday rotator mentioned above. To be specific, for example, this may use a Farad ay rotator with a configuration wherein an external magnetic field is applied using a permanent magnet and an electromagnet, and an amount of transmitted light is controlled by positioning a polarizer and an analyzer in front and behind the Faraday rotator in the light beam direction, and varying the external magnetic field by the electromagnet.